It is known in the art that electrochemical cells having an anode of lithium or other Group I metal have high energy densities, high voltages, wide temperature operating ranges, long shelf-life and relatively low cost.
In such cells, the type now commonly referred to as the "liquid cathode" cell are of particular interest. These cells are described, for example in U.S. Pat. No. 3,926,669 and in British Pat. No. 1,409,307. In accordance with the teachings of the aforementioned patents, the electrolyte comprises an oxyhalide, for example thionyl chloride.
One attractive system is provided through the use of thionyl chloride, a solute of lithium aluminum tetrachloride, a lithium anode and a cathode current collector, which may be for example compressed carbon black. Despite the numerous advantages envisioned for lithium liquid cathode batteries, it has been found that problems occur with these cells. A particular problem has been identified as that of voltage delay particularly occurring when the batteries are stored at elevated temperatures. The voltage delay problem is characterized by two features; to wit, a dip in voltage below a defined cutoff voltage and the time required for the voltage to rise back to the cutoff voltage. The problem is believed to result from passivation of the lithium anode due apparently to reactions between the lithium anode and constituents of the electrolyte.
In accordance with U.S. Pat. No. 4,170,693 it is proposed to minimize the voltage delay problem by coating the lithium anode with a cyanoacrylate polymeric coating.
The prior art is not clear as to the desirability of low reactivity between a protective coating and the lithium base. Rao, in U.S. Pat. No. 4,056,885, teaches that laminating (coating) lithium with aluminum results in a lithium-aluminum alloy due to the reactivity of aluminum with lithium. In U.S. Pat. No. 4,002,492 Rao also teaches that such a lithium-aluminum alloy has many desirable characteristics as use for anode materials. On the other hand, Catanzarite discloses that the material used in protective coatings of lithium should have low reactivity with the lithium base in U.S. Pat. No. 4,170,693. So on the point of reactivity between coatings and the lithium base the prior art teaches that beneficial effects can be obtained with either high or low reactivity coatings and so it is not a priori obvious as to which type of reactivity to choose for a protective coating for lithium.
The prior art is also not clear as to what the reactivity should be of materials added to electrolytes. Rao, in U.S. Pat. No. 4,002,492, teaches that an electrolyte salt considered for incorporation into the electrolyte should have low reactivity towards lithium. But Driscoll, in U.S. Pat. No. 4,093,784, teaches that it is desirable to place in the thionyl chloride electrolyte a calcium-containing salt in order that the lithium anode reacts with the calcium salt.